8/10/2019 CCS#7 Signalling http://slidepdf.com/reader/full/ccs7-signalling 1/23 Common Channel Signaling System No.7 __________________________________________________________________________________ _ 7 COMMON CHANNEL SIGNALLING SYSTEM NO. 7 OVERVIEW OF SIGNALLING SYSTEMS One of the major factors influencing the development of signaling systems is the relationship between signaling and the control function of exchanges. Early telecommunication networks used analogue step-by-step exchanges. In such systems, the control and switch functions are co-located, and when a call is made, the signaling and traffic follows the same path within the exchange. his is known as !hannel "ssociated #ignaling $!"#%. In this case, the signaling and traffic also follows the same path external to the exchange, i.e. on the transmission link. he next stage trough, which the exchanges evolved, is shown in &ig. '.(. In such exchange the control mechanism for setting-up and releasing calls is separated from the switch block. he techni)ue allows much more flexibility in controlling calls and it also reduces costs. "gain, !"# systems are typically associated with this type of exchange. *hereas signaling information is carried on the same path as it is associated speech circuit external to the exchange. he two are separated within the exchange. his is shown in &ig. + in which the speech traffic circuits $denoted by solid lines% are routed by the switch block but the signaling information $denoted by dotted lines% is routed via the control function. etween Exchanges " and , the signaling and traffic are carried over the same path. his approach was primarily designed to allow optimiation of functions within exchanges, but its effectiveness is constrained by the need to combine signaling and speech traffic external to the exchange. *ith !ommon !hannel #ignaling $!!#% systems, the philosophy is to separate the signaling path from the speech path. he separation occurs both within the exchange and external to the exchange $&ig. +(%, thus following optimiation of the control processes, switch block and signaling systems. &ig. +( illustrates that, in a !!# environment, the switch block routs the speech paths as before, and however, a separate path internal to the exchange routs the signaling $denoted by a dotted line%. his approach allows maximum flexibility in optimiing exchange and signaling development. he approach gains maximum benefits when adopted in parallel with the introduction of digital exchange and digital transmission systems. !!# system being particularly efficient in these circumstances. Advantages Of Common Canne! S"gna!!"ng !ommon !hannel #ignaling is being adopted throughout the world in national and international networks for numerous reasons. he main reasons are/ • he rapidly changing control techni)ues of exchanges. • he limitations of !"# systems. • he evolutionary potential of !!# systems. '' 00"I, 1abalpur
he separation of !!# from traffic circuits, and the direct inter-connection of
exchange processors, is the early steps in establishing a cohesive !!# network to allow
unimpeded signaling transfer between customers and nodes and between nodes in the
network. he concept of a cohesive !!# network opens up the opportunity for theimplementation of a wide range of network management administrative, operation and
maintenance function. " major example of such function is the )uasi-associated mode of
operation. his mode of operation provides great deals of flexibility in network security,
reduces the cost of !!# on small traffic routes and extends the data-transfer capabilities for
non-circuit related signaling.
!"# systems posses limited information-transfer capability due to/
• he restricted number of conditions that can be applied $e.g. the limit variation that
can be applied to a 4.!. loop or the limited number fre)uency combinations that can
be implemented in a voice fre)uency system%• he limited number of opportunities to transfer signals $e.g. it is not possible to
transmit voice-fre)uency signals during the conversation phase of a call without
inconveniencing the customers or taking special measures.%
5either of these restrictions applies to !!#. he flexible message-based approach
allows a vast range of information to be defined and the information can be during any stage
of a call. 2ence, the repertoire of !!# is far greater than channel-associated versions and
messages can be transferred at any stage of a call without affecting the calling and called
subscribers.
!!# systems transfer signals very )uickly, i.e. at 6' 7bps. his speedy signaling also
permits the inclusion of far more information without an increase in post dialing delay.
echni)ues used in modern !!# system can further improve the flexibility provided
to customers. 89ser-to-user8 signaling and end-to-end signaling techni)ues are used whereby
messages can be transferred from one customer to another without undergoing a full analysis
at each exchange in the network. *hilst forms of end-to-end signaling are possible using
!"# systems, the techni)ue can be more efficiently implemented with !!# systems.
One of the problems that prompted the development of !!# systems was 8speech
clipping8 in the international network. In some case !"# systems, it is necessary to split the
speech path during call set-up to avoid tones being heard by the calling customer. his results
in a slow return of the answer signal and, if the called customer starts speaking immediatelyafter answer, then the first part of the statement by the called customer is lost. "s the first
statement is usually the identity of the called customer, this causes a great deal of confusion
and inconvenience. !!# systems avoid the problem by transferring the answer signal )uickly.
"s a result of the processing ability of !!# systems, a high degree of reliability can
be designed into the signaling network. Error detection and correction techni)ues can be
applied which ensure reliable transfer of uncorrupted information. In the case of an
intermediate exchange failure, re-routing can take place within the signaling network,
enabling signaling transfer to be continued. *hile these features introduce extra
re)uirements, the common channel approach to signaling allows a high degree of reliability
to be implemented economically.
" major restriction of !"# is the lack of flexibility, e.g. the ability to add newfeatures is limited. One factor that led to the development of !!# was the increasing need to
add new features and respond to new network re)uirements. 0esponses to new re)uirements
in !!# can be far more rapid and comprehensive than for channel associated versions.
!!# systems are not just designed to meet current needs. hey are designed to the as
flexible as possible in meeting future re)uirements. One way of achieving the objective is to
define modern !!# systems in a structured way, specifying the signaling system in a number
of tiers. he result is flexibility signaling system that reacts )uickly to evolving re)uirements
and future services can be incorporated in a flexible and comprehensive manner. !hanges to
existing services can be implemented more )uickly and at lower cost than with !"# systems.
Ove%v"e& Of S"gna!!"ng S'stem No. 7
#ignaling #ystem 5o. : $!!#:% is a message based signaling system between #tored
;rogram !ontrolled $#;!% switches. *here the intermediate nodes may be used a #ignal
ransfer ;oints $#;s% !!#: network can be used for transmitting call related messages, as
well as show speed data packets between I#45 users. he #ignaling !onnection !ontrol ;art
$#!!;% enables it to act like a packet network. hus it is an important pre-re)uisite to
<evel + defines the functions that are relevant to an individual signaling link,
including error control and link monitoring. hus, level + is responsible for the reliable
transfer of signaling information between two directly connected signaling points. If errors
occur during transmission of the signaling information, it is the responsibility of level + to
invoke procedures to correct the errors. #uch characteristics can be optimied without
affecting the rest of the signaling system, provided that the interfaces to level ( and = remain
constant.
LEVEL -
he functions that are common to more than one signaling link, i.e. signaling network
functions, are defined in level = / these include 8message handling8 functions and 8signaling
network management8 functions. *hen a message is transferred between two exchanges,
there are usually several routes that the message can take including via a signal-transfer point.
he message-handling functions are responsible for the routing of the messages through the
signaling network to the correct exchange. #ignaling network management functions control
the configuration of the signaling network. hese functions include network reconfiguration
is response to status changes in the network. &or example, if an exchange within the signaling
network fails, the level = of !!#: can re-route message and avoid the exchange that has
failed.
Message T%ansfe% )a%t MT)/
<evel ( to = constitutes a transfer mechanism that is responsible for transferringinformation in messages from one signaling point to another. he combination of level ( to =
is known as the message transfer part $>;%. he >; controls a number of signaling
message links and network management functions to ensure correct delivery of messages.
his means that the messages are delivered to the appropriate exchange in an uncorrupted
form and in the se)uence that they were sent, even under failure conditions in the network.
LEVEL 0
<evel ' comprises the 8user parts8. he meaning of the messages transferred by the
>; and the se)uence of actions for a particular application $e.g. telephony% is defined by the
8user parts8. " key feature is that many different user parts may use the standardied >;.
2ence, if new re)uirements arise, that had not been foreseen previously, the relevant user partcan be enhanced $or a new user part derived% without modifying the transfer mechanism or
affecting other user parts. hree user parts have been defined, the elephone 9ser ;art $9;%,
the I#45 9ser ;art $I#9;% and the 4ata 9ser ;art $49;%. "long with #!!;, which provides
end-to-end signaling capability, >; constitutes the 5etwork #ervices ;art $5#;% which
provides the 5etwork <ayer functionalities of the O#I model. he user parts of 5#; are
Operations and >aintenance "pplication ;art $O>";% and >obile "pplication ;art $>";%.
S"gna!!"ng Conne*t"on Cont%o! )a%t SCC)/
he #ignaling !onnection !ontrol ;art $#!!;% has the functions of the network as
well as the transport layers of the !!#: protocol stack. ogether with the >;, it provides
the O#I transport layer capabilities. 9nlike >;, which provides only data gram service,
#!!; provides connection-oriented, and connection less services as well.
hus, while >; is sufficient for circuit switched application like 9; and I#9;, for non-circuit related applications, such a database )uerying, the enhanced addressing capability
of #!!; is re)uired. #!!; has a uni)ue scheme of addressing and routing based on Alobal
itles. #!!; utilies the services of >; to route its payload from one node to other.
In addition to routing transaction related messages submitted by the ransaction
!apabilities "pplication ;art $!";%, #!!; also segments and se)uences large !";
messages to fit into the >; packet sie. "t the distant node it is the responsibility of the peer
#!!; to re-assemble the segmented message.
T%ansa*t"on Ca1a2"!"t"es A11!"*at"on )a%t TCA)/
!"; is an application part in the !!#: stack and is responsible for establishingdialogue with remote databases. It carries the date of higher layers like I5"; and >"; and
invokes remote operations. "n operation at remote and re)uires a series of )ueries and
response as part of a !"; dialogue.
Management of a d"a!og(e %e3("%es#
• Establishing a dialogue.
• !ontinuing the dialogue.
• erminating the dialogue.
• >aintaining the integrity of each dialogue in case of multiple dialogue scenarios by
assigning uni)ue transaction ids to each dialogue.• Invoking remote operation and managing the operation.
!"; layer is a compound layer in the sense that it is composed of two sub-layers
namely, ransaction #ub layer $#<% and !omponent #ub layer $!#<%. ransaction sub layer
is responsible for establishing, managing and maintaining the integrity of the dialogue
whereas !omponent sub layer is responsible for packing the upper layer message into a
component and assigning an invoke I4 to the component.
*hen !!#: is specified as a signaling system, level ' specifies a number of call-
control functions. Indeed, the circuit-related mode of !!#: is so closely associated with
controlling the set-up and release of physical circuits that it is essential that some aspects of call control are defined within the user part specification in order to optimie the procedures
that are adopted.
A11!"*at"on Of Te Leve! St%(*t(%e
he application of the level structure is illustrated in &ig.+= Exchanges " and are
directly connected by speech circuits $denoted by the sold lines connecting the respective
switch blocks%. " signaling link is also available between Exchanges " and $denoted by the
dotted line%. It is shown that level ' $the user part% is closely associated with the control
If the control function of exchange " needs to communicate with the control function
of Exchange $e.g. to initiate the set-up a speech circuit between the exchanges%, the control
function in Exchange " re)uest the level ' functions to formulate an appropriate message.
<evel ' then re)uests the message-transfer part $level ( to =% to transport the message toexchange . <evel = analyses the re)uest and determines the means of routing the message of
exchange . the message is then transported via levels + and (.
9pon receipt of the message by the >; of exchanges , levels ( and + deliver the
message to level =. <evel = at exchange recognies that the message has arrived at the
correct exchange and distributes the message to the appropriate user part at level '. <evel ' in
exchange then interacts with the control function to determine the appropriate action and
response. If problems arise in the transmission process between exchanges " and . causing
message corruption, the level + functions are responsible for detecting the corruption and
retransmitting the information. If the signaling link between exchanges " and is not
available $e.g. link has failed%, the level = functions are responsible for re-routing theinformation through the signaling network to exchange .
Int%od(*t"on
!ommunication networks generally connect two subscriber terminating e)uipment
units together via several line sections and switches for message exchange $e.g. speech, data,
text or images%. !ontrol information has to be transferred between the exchanges for call
control and for the use of facilities. In analog communication networks, channel-associated
signaling systems have so far been used to carry the control information. &ault free operation
is guaranteed with the channel associated signaling systems in analog communication
networks, but the systems do not meet re)uirements in digital, processor and controlled
communication network. #uch networks offer a considerably larger scope of performance ascompared with the analog communication networks due, for instances, to a number of new
services and facilities. he amount and variety of the information to be transferred is
accordingly larger. he conventional channel associated signaling systems can no longer
economically transport the information. &or this reason, a new efficient signaling is re)uired
in digital, processor-controlled communication networks.
• #ignaling on separate signaling linksD the bit rate of the circuit is therefore exclusively
for communication.
• #ignaling links always available, even during existing calls.
• 9se of the signaling links of transferring user data also
• 9sed on various transmission media
• cable $copper, optical fiber%
• radio relay
• satellite $up to + satellite links%
• use of the transfer rate of 6' kbits typical in digital networks
• used also for lower bit rates and for analog signaling links if necessary
• "utomatic supervision and control of the signaling network.
S"gna!!"ng Net&o%+
In contrast to channel-associated signaling, which has been standard practice untilnow, in !!#: the signaling messages are sent via separate signaling links $see fig.+'%. One
signaling link can convey the signaling messages for many circuits.
he !!# signaling links connect signaling points $#;s% in a communication network.
he signaling points and the signaling links form an independent signaling network, which is
overlaid over the circuit network.
F"g # 0 #ignaling via a common channel signaling link
S"gna!!"ng )o"nts S)/
" distinction is made between signaling points $#;% and signaling transfer points
$#;%.
he #;s are the sources $originating points% and the sinks $destination points% of
signaling traffic. In a communication network these are primarily the exchanges.
he #;s switch signaling messages received to another #; or to a #; on the basis
of the destination address. 5o call processing of the signaling message occurs in a #;. "
#; can be integrated in a #; $e.g. in an exchange% or can form a node of its own in the
signaling network. One or more levels of #;s are possible in a signaling network, accordingto the sie of the network.
"ll #;s in the signaling network are identified by means of a code within the
framework of a corresponding numbering plan and therefore can be directly addressed in a
signaling message.
S"gna!!"ng L"n+s
" signaling link consists of a signaling data link $two data channels operating together
in opposite directions at the same data rate% and its transfer control functions. " channel of an
existing transmission link $e.g. a ;!> = link% is used as the signaling data link. Aenerally,
more than one signaling link exists between two #;s in order to provide redundancy. In thecase of failure of a signaling link, functions of the !!#: ensure that the signaling traffic is
rerouted to fault free alternative routes. he routing of the signaling links between two #;s
can differ. "ll the signaling links between two #;s are combined in a signaling link set.
S"gna!!"ng Modes
he different signaling modes can be used in the signaling networks for !!#: vi.
associated mode and )uasi-associated mode.
In the associated mode of signaling, the signaling link is routed together with the
circuit group belonging to the link. In other words, the signaling link is directly connected to
#;s which are also the terminal points of the circuit group $see fig.+3% his mode of recommended when the capacity of the traffic relation between the #;s " and heavily
utilied.
F"g # 4 "ssociated mode of #ignaling
In the )uasi -associated mode of signaling, the signaling link and the speech circuit
group run along different routes, the circuit group connecting the #; " directly with the #; .
&or this mode the signaling for the circuit group is carried out via one or more defined #;s
$see fig.+6%. his signaling mode is favorable for traffic relations with low capacity
utiliation, as the same signaling link can be used for several destinations.
he route defined for the signaling between an originating point and a destination
point is called the signaling route. he signaling traffic between two #;s can be distributedover several different signaling routes. "ll signaling routes between two #;s are combined in
a signaling route set.
Net&o%+ St%(*t(%e
he signalling network can be designed in different ways because of the two
signalling modes. It can be constructed either with uniform mode of signalling $associated or
)uasi-associated% or with a mixed mode $associated and )uasi-associated%.
he worldwide signalling network is divided into two levels that are functionally
independent of each other, an international level with an international network and a national
level with many national networks. Each network has its own numbering plans for the #;s.
)!ann"ng As1e*ts
Economic, operational and organiational aspects must be considered in the planning
of the signalling network for !!# :. "n administration should also have discussions with the
other administrations at an early stage before !!# : is introduced in order to make decisions,
he #IO contains additional address information. 9sing the #I, the destination >;
identifies the 9; for which the message is intended. he 5I, for example, enables a message
to be identifies as being for national or international traffic.
<##9s and &I#9s re)uire no routing label as they are only exchanged between level +
of adjacent >;s.
he message sent from a user to the >; for transmission contains/ the user
information, the routing label, the #I, he >I and a <I. he processing of a user message to
be transmitted in the >; begins in level =.
he >; is responsible for $a% transmitting and $b% receiving #9s $c% for correcting
transmission errors, $d% for the signalling network management and $e% for the alignment. Its
functions are spread over the functional levels, (, +, and =.
he message routing $level =% determines the signalling link on which the user
message is to be transmitted. o do this, it analyes the 4;! and the #<# field in the routing
label of the user message, and then transfers the message to the appropriate signalling link
$level +%
he transmission control $level +% assigns the next  and the &I to the user
message. In addition, it includes the #5 and the I as an acknowledgement for the last
received >#9. he transmission control simultaneously enters the part of the >#9 formed
so far in the transmission and retransmission buffers. "ll >#9s to be transmitted are stored in
the retransmission buffer until their fault-free reception is acknowledged by the receive side.
Only then they are deleted.
he check bit and flag generator $level +% generates !7s for safeguarding against
transmission errors for the >#9 and sets the flag for separating the #9s. In order that any
section of code identical to the flag $((((((% occurring by chance is not mistaken for the
flag, the user message are monitored before the flag is added to see if five consecutive ones
$(% appear in the message. " ero $% is automatically inserted after five consecutive (s. On
the receive side the ero following the five (s is then automatically removed and the user
message thereby regains its original coding.
he check-bit stream along a signalling data link is received in level ( and transferred
to level +. &lag detection $level +% examines the received bit stream for flags. he bitse)uence between two flags corresponds to one #9. he alignment detection $level+%
monitors the synchronism of transmit and receive sides with the bit pattern of the flags.
9sing the !7s also transmitted error detection $level +% checks whether the #9 was
correctly received. " fault free #9 is transferred to the receive control, while a faulty #9 is
discarded. he reception of a faulty #9 is reported to error rate monitoring, in order to keep a
continuous check on the error rate on the receive side of the signalling link. If a specified
error rate is exceeded, this is reported to the signalling link status control by error rate
monitoring. he signalling link status control then takes the signalling link out of service and
he receive control $level +% checks whether the transferred #9 contains the expected
 and the expected &I. If this is the case and if it is a >#9, the receive control transfers
the user message to level = and causes the reception of the >#9 to be positively
acknowledged. If the  of the transferred >#9 does not agree with that expected, thereceive control detects a transmission error and causes this and all subse)uent >#9 to be
retransmitted $see subheading G!orrection of transmission errorsG%.
he message discrimination $level =% accepts the correctly received user message. It
first determines whether the user message is to be delivered to one of the immediately
connected 9;s or to be transferred to another signalling link $)uasi-associated message%. his
preselection is achieved in the message discrimination by evaluation of the 4;!. " user
message that only passes through a #; $#;% is transferred by the message discrimination to
the message routing, where itHs treated as a user message to be transmitted.
If a received user message is intended for one of the connected 9;s $#;%, it istransferred to message distribution $level =%. he message distribution evaluates the #IO,
thereby determining the 9; concerned, and delivers the user message there.
S"gna!!"ng Net&o%+ Management
he signalling network management is a function of level =. It controls the operation
and the interworking of the individual signalling links in the signalling networks. o this end,
the signalling network management exchanges messages and control instruction with the
signalling links of level +, sends message to the 9;s and works together with the signalling
network management in adjacent #;s. &or the interworking with other #;s the signalling
network management uses the transport function of the >;. >anagement messages aretransferred in >#9s like user messages. &or discrimination, the management messages have
their own #I. he signalling network management contains = functions blocks.
a. he signalling link management controls and monitors the individual signalling links. It
receives the messages concerning the alignment and status of the individual signalling
link, or concerning operating irregularities and effects any changes in status which may
be necessary. It addition, the signalling link management controls the putting into service
of signalling links, including initial alignment and automatic realignment of signalling
links after failures or alignment losses due to persistent faults. If necessary, the signalling
link management transfers messages to the signalling traffic management or receives
instructions from there. b. he signalling route management controls and monitors the operability of signalling
routes. It exchanges messages with the signalling route management in the adjacent #;s
for this purpose. he signalling route management receives, for example, messages
concerning the failure or reavailability of signalling routes or the overloading of #;s. In
cooperation with the signalling traffic management, it initiates the appropriate actions in
order to maintain the signalling operation to the signalling destinations involved.
c. he signalling traffic management controls the diversion of the signalling traffic from
faulty signalling links or routes to fault-free signalling links or routes. It also controls the
load distribution on the signalling links and routes. o achieve this it can initiate the
!hangeover, on failure of a signaling link the signaling traffic management switches
the signaling traffic from the failed signaling link to a fault-free signaling link
!hange backD when signaling link becomes available again after a fault has been
corrected, the signaling traffic management reverses the effect of the changeover. 0eroutingD when #; can no longer be reached on a normal route, the signaling traffic
management diverts the signaling traffic to a predefined alternative route.
*hen overloading occurs, the signalling traffic management sends messages to the
users in its own #; in order that they reduce the load. he management also informs the
adjacent #;s of the overloading in its own #; and re)uests them to also reduce the load.
he signaling traffic management accomplishes its functions by
0eceiving messages from the signaling link and signaling route management.
#ending control instructions to signaling link and signaling route management.
4irectly accessing the signaling links, e.g., during emergency alignment.
>odifying the message routing on failure of signaling routes.
Exchanging management message with the signaling traffic management in adjacent
#;s.
:se% )a%ts :)/
"s earlier level ' functions, which include formatting of messages based on the
applications, are allotted to 9;s. Each 9; provides the functions for using the >; for a
particular user type. #ome of the 9ps as currently specified by the !!I are/
elephone user part $9;%
Integrated services digital network user part $I#45-9;%
he signaling connection control part $#!!;%
he transaction capabilities application part $!";%
&or Intelligent 5etwork $I5% application, Intelligent "pplication ;art $I5";% and
!"; are used. #!!; forms the interface between these 9;s and >;.
&ig. =( #hows the users of the >; as well as their relationship to one another and to
the >;. !!#: can be adapted to all re)uirements due to the modular structures. Expansion
for future application is also possible. Each !!# user can specify its own 9;, for example,
the mobile user part $>9;% is #iemen8s own specification for the mobile telephone network
